1. Field of the Invention
This invention relates to optical fibers and processes for their fabrication.
2. Description of the Prior Art
The transmission characteristics of an optical fiber must be of primary concern if the fiber is to be practical for transmitting information over long distances. Phenomena which interfere with the transmission efficiency of an optical fiber fall into two broad categories. The first involves the absorption of light within the transmitting material. Such absorption results in loss of signal and is directly related to the distance over which a signal can be transmitted without being reprocessed. Improved fabrication and material techniques have resulted in optical fibers with losses less than 5 dB/km at useful wavelengths, making possible repeater-free transmission over distances as great as 5 km.
The second optical fiber transmission property which is of concern involves the ability of the fiber to transmit pulses of light while maintaining their initial pulse width to as high a degree as possible. The bandwidth of the transmission line is directly related to this property. Degradation of the pulse width results from a number of different physical phenomena. One is material dispersion -- the dependence of the velocity of light in the material on the frequency of the light being transmitted. A second and usually more significant phenomenon which also results in degradation of the pulse width is associated with the mode structure of the radiation traversing the fiber. Under certain conditions of index of refraction, fiber diameter, and wavelength, the fiber will suport only one mode. However, under other and sometimes more desirable conditions a multitude of modes may be simultaneously supported by the fiber. One may associate with each such mode a different ray path through the fiber. "Short length" paths proceed directly down the center of the fiber. "Long length" paths may reflect off the fiber walls numerous times. A different traversal time may be associated with each mode and, consequently, a pulse propagating through the fiber in a multitude of modes will be broadened.
At least two techniques have been found to reduce this "mode dispersion". One involves fabricating the fiber with high frequency longitudinal variations in its optical properties, e.g., index of refraction. Such variations are found to result in efficient conversion between the guiding modes, yielding an average traversal time and improved pulse response. The second technique for reducing mode dispersion involves grading the index of refraction of the fiber along the radial direction from a maximum at the center of the transmitting core to a minimum at its perimeter. In such a graded fiber, the velocity of light is highest near the perimeter of the transmitting core and lowest near its center. Consequently, the long path-length modes, which spend more time near the fiber perimeter, have higher average velocities, and hence traversal times which are more nearly equal to the traversal time of the short path-length modes. In this manner, the pulse disersion is minimized.
A continuous radial gradation in index of refraction is approximated by a fiber which may be defined in terms of coaxially nested tubular regions each of different index of refraction. However, in such fibers if the radial width of each tubular region is greater than the wavelength of the light being transmitted then the theoretical improvement associated with a continuous radial gradation can only be approached and some pulse broadening still occurs. Any pulse broadening that remains increases directly with the length of the fiber, as does other broadening associated with mode dispersion.
In a commonly assigned application by S. E. Miller, (Ser. No. 710,137, filed July 30, 1976) it is shown that a low frequency longitudinal variation in index of refraction, when coupled with a nonuniform cross sectional index of refraction, yields efficient nonadiabatic mode conversion. Such mode conversion is distinct from the mode conversion obtained in prior fibers having only high frequency longitudinal variations in their optical parameters. Specifically, the prior high frequency longitudinal variations (of the order of from 1 to 10 mm) result in adiabatic mode conversion and require no cross sectional nonuniformity in the index of refraction to effect the requisite mode conversion. Nonadiabatic mode conversion, on the other hand, requires a nonuniform cross sectional index configuration coupled with a longitudinal variation in index of refraction of from 0.1 to 400 meters in period in order to effect efficient mode conversion. Such low frequency longitudinal variations lend themselves much more readily to current fabrication processes. As in the prior art fibers, resulting mode conversion occurs primarily between the guided modes rather than between the guided and radiating modes. Radiation losses are thereby kept to a minimum and the width of a pulse traversing the fiber increases directly with the square root of the fiber length, rather than directly with fiber length as is the case without efficient mode conversion.
Although any nonuniformity in cross sectional index of refraction when coupled with a low frequency longitudinal variation is sufficient to obtain nonadiabatic mode conversion, particular advantages result when the cross sectional index of refraction is graded from a maximum at the center to a minimum at the fiber perimeter. Under such circumstances, one gains the advantage of having both a radially graded fiber and a mode-mixing fiber in one single configuration.
Nonadiabatic mode conversion may be introduced into a radially graded fiber by fabricating each of the associated tubular regions with longitudinal variations in index of refraction. The variations have a period of from 0.1 to 400 meters. The longitudinal variations need only be approximately periodic and should be in approximate antiphase with the longitudinal variations of the adjacent tubular regions. Under such a circumstance, the minimum index of refraction points of a given n.sup.th tubular region should occur adjacent to, and approximately equal in value to, the maximum index points of the (n+1).sup.th tubular region of greater radius. Departures from periodicity or from the required antiphase relationship are not critical as long as there are regions where the n.sup.th and (n+1).sup.th regions have approximately equal index of refraction. The more frequent such regions and the greater the equality of the indices in these regions, the more efficient is the mode conversion. Consequently, a preferred structure is one in which the longitudinal variation in each tubular region is periodic and in antiphase relationship with the tubular region adjacent to it.
In the above-mentioned commonly assigned application by S. E. Miller, such fibers that have both a radially graded index configuration and longitudinal variations in index of refraction are fabricated using standard deposition processes. In such deposition, a glass cylindrical starting member may be coated either externally or internally with layers of glass material using any one of the standard deposition processes. Each layer is deposited individually by rotating the glass cylindrical starting member and simultaneously traversing it with a glass deposition device. The material composition being deposited is varied slowly compared to the traversal speeds in such a manner that ringed regions of index of refraction are obtained for each layer.
It should be emphasized that throughout the discussion of this invention the description of the fiber in terms of layers is only for greater ease of understanding. The fiber is viewed as comprising tubular regions but need not in fact have physically discernible layers. For example, the MCVD process may result in the fabrication of a fiber without physically discernible layers, and if such layers do inadvertently occur they are not germane to the efficacy of the process.